Many federal facilities may have water uses that can be met with non-potable water from alternative water sources. Potentially available alternative water sources for federal sources include municipal-supplied reclaimed water, treated gray water from on-site sanitary sources, and storm water.
On-site alternative water sources are most economical if included in the original design. Common uses for these sources include landscape irrigation, ornamental pond and fountain filling, cooling tower make-up, and toilet and urinal flushing.
Municipal-Supplied Reclaimed Water
Municipal supplied reclaimed water has been treated and recycled for non-potable use. This water is often available at a significantly lower rate than potable water; however, use of reclaimed water is often restricted by local codes.
Traditionally, centralized municipal sewage treatment facilities have been the primary sources of water disposal for federal facilities. However, heightened concerns about water supply availability have encouraged facility managers to consider on-site recycling of wastewater or gray water.
Treated Gray Water
Sanitary gray water is generated by bathroom sinks, showers, and clothes washing machines. These water sources can contain pathogens. Non-sanitary gray water is generated by industrial processes or equipment such as reverse osmosis reject water and cooling tower condensate. These water sources can contain chemicals, minerals, and solids.
In a typical gray water recycling system, water that would normally be discharged for municipal sewage treatment is collected, treated to remove suspended solids and contaminants, and reused. On-site wastewater recycling applications are currently found in states with persistent drought conditions. However, all arid, semiarid, and coastal areas that have experienced water shortages, as well as major urban areas where sewage treatment plants are overloaded and expansion is constrained, are potential candidates for on-site recycling. The recycled water is typically used as flush water for toilets and urinals, landscape irrigation, supply water for ornamental ponds, and make-up water for cooling towers.
At its most basic, gray water treatment consists of removing suspended solids from the water. Filtering with no additional treatment may be applicable for rinse water from laundries or car washes and air handler condensate. At its most sophisticated, treatment may consist of biological treatment with membrane filtration, activated carbon, and ultraviolet light or ozone disinfection to destroy pathogens. The basic gray water system includes storage tanks, color-coded piping, filters, pumps, valves, and controls.
Rainwater harvesting captures, diverts, and stores rain water for later use. Captured rainwater is often used in landscaping, because the water is free of salts and other harmful minerals and does not have to be treated. It is also useful in attracting and providing water for wildlife. Rainwater harvesting can also help prevent flooding and erosion, turning storm water problems into water supply assets by slowing runoff and allowing it to soak into the ground. Reducing run-off also helps reduce contamination of surface water with sediments, fertilizers, and pesticides in rainfall run-off.
Rainwater can be collected in cisterns and used with little or no treatment for a variety of non-potable purposes. The major components of a rainwater harvesting system include:
- Catchment area, roof, or surfaces upon which the rain falls
- Gutters and downspouts to carry the water to storage
- Leaf screens to remove debris
- Cisterns/storage tanks to store the harvested rainwater
- Conveyances to deliver the stored water either by gravity or pump
- Water treatment system to settle, filter, and disinfect the water, if required.
Like rainwater, storm water can be harvested and reused for washdown, cooling tower make-up or process water, dust suppression, and vehicle washing. Storm water harvesting differs from rainwater harvesting because the runoff is collected from drains or creeks, rather than roofs. The characteristics of storm water harvesting and reuse schemes vary considerably between projects, but most schemes include collection, storage, treatment, and distribution.
Storm water is generally collected from a drain, creek, or pond and then stored temporarily in dams or tanks to balance supply and demand. Storage can be online (constructed on the creek or drain) or offline (constructed some distance from the creek or drain). Captured water is typically treated to reduce pathogens and pollution levels through the use of constructed wetlands, sand filters, and disinfection techniques including chlorine, ultraviolet radiation, and membrane filtration. The degree of treatment required depends on the proposed use and the level of public exposure.
Urban storm water harvesting and reuse is a relatively new field of water management and most of the projects constructed to date have been pilot projects in drought areas.
Successful storm water harvesting and reuse plans need specialist input from a number of areas, including storm water management, water supply management, environmental management, and public health. The potential limitations and disadvantages to storm water harvesting and reuse include variable rainfall patterns, environmental impacts of storages, potential health risks, and high relative unit costs of treated storm water. Although this is not a practice that would be feasible at many federal facilities, storm water harvesting and reuse is an emerging practice that may be useful in specific situations.
To develop an efficient and successful reclaimed water project you must have a reliable source of wastewater of adequate quantity and quality to meet your non-potable water needs. These projects are economically more viable when:
- The cost of water is high
- There is a lack of high-quality freshwater supply
- Reuse is the most cost-effective way to dispose of wastewater effluent
- There are local policies that encourage the use of reclaimed water, or water efficiency.
Operation and Maintenance Options
To maintain water efficiency in operations and maintenance, Federal agencies should:
Identify potential non-potable water use while reviewing current water use practices. The use of non-potable water is generally most cost effective when included in the design of new facilities.
Consult with experts in the field. Your first resource should be local or headquarters engineers, but do not rule out input from contractors or other government agencies.
Facilities using alternative on-site water sources must comply with all applicable backflow prevention requirements.
Municipal Supplied Reclaimed Water
Municipal supplied reclaimed water pipes must be color coded with purple tags or tape according to standards set by the American Water Works Association to minimize cross-connection problems.
Signs should be used liberally to indicate that reclaimed water is non-potable. Place them in public places such as in front of a fountain and on valves, meters, and fixtures.
Keep the pressure of reclaimed water 10 psi lower than potable water mains to prevent backflow and siphonage in case of accidental cross-connection.
Run reclaimed water mains at least 12 inches lower in elevation than potable water mains and horizontally at least five feet away.
Review the quality of reclaimed water to ensure there will be no harmful effects, such as salt buildup, from long-term use.
Gray Water Recycling Systems
The pathogenic organisms in sanitary gray water must not come into contact with either humans or animals. This can be accomplished by treating the water to eliminate pathogens or avoiding their introduction into water by not mixing sanitary gray water with any potable water source. Human exposure can be prevented by not collecting or storing it in an open container.
Sanitary gray water used for irrigation should not be applied through a spraying device, but rather injected directly into the soil through drip irrigation. Drip irrigation allows you to receive the benefits of using recycled water and at the same time avoid contaminating animals, humans, and edible plants.
If you install a gray water recycling system, consider using biodegradable cleaning products that do not contain sodium, chlorine, or boron. Cleaning products that contain high chemical levels may make their way into the gray water recycling system and could poison plants or damage soil through the buildup of inorganic salts.
Rain or excessive irrigation could cause ground saturation and result in pools of gray water on the surface. To help eliminate this situation, turn the gray water system off and divert the gray water to the sanitary sewer line during rainy periods.
Maintenance programs for a gray water system must include the following steps, all of which must be performed regularly:
- Inspecting the system for leaks and blockages
- Cleaning and replacing the filter bimonthly
- Replacing the disinfectant
- Ensuring that controls operate properly
- Periodically flushing the entire system.
Retrofit and Replacement Options
The following retrofit and replacement options help federal agencies implement gray water recycling systems and rain water harvesting:
Gray Water Recycling System
The use of on-site wastewater recycling systems should be considered when constructing new buildings. Even though many of these systems are costly to purchase, the payback period in savings from discharging less wastewater can be as little as 10 years or less.
Gray water systems must be installed in accordance with local plumbing codes and by professional, licensed plumbing contractors. Installing a gray water system requires the retrofitting of existing plumbing and all alterations to the plumbing system must be approved by local authorities.
All counties and cities that permit gray water recycling require building inspections to inspect sites and, after the installation, verify compliance and proper operation of the gray water system.
Local authorities may require that gray water supply systems be clearly distinguished from potable water supplies. Methods of doing so include extensive labeling of the system or the use of different piping materials for the different systems. All gray water outlets must be clearly labeled to indicate that they dispense non-potable water. Local codes may also require marking gray water supplies by adding biodegradable dye. Additionally, backflow preventers also must be installed to ensure the proper separation of potable water and gray water supply system.
For buildings with slab foundations, recoverable gray water may be limited to washing machine discharge because most drain pipes, such as for sinks, are buried beneath the slab and thus not easily accessible without a significant additional expense.
For buildings with perimeter foundations, gray water may be recoverable from most sources due to accessibility to piping from crawl spaces.
Depending on the gray water source, application, recycling scheme, and economics, one gray water treatment method (e.g., media filtration, collection and settling, biological treatment units, reverse osmosis, sedimentation/filtration, physical/chemical treatment) may be more appropriate than another.
Rainwater collection and distribution systems can be incorporated into almost any existing site, although it is easier to incorporate them into new construction.
According to The Texas Manual on Rain Water Harvesting, you can collect 600 gallons of water per inch of rain per 1,000 square feet of catchment area. To determine how much water you could collect in a year:
- Calculate the roof catchment area
- Multiply the collection area in square feet by 0.6 gallons per square foot per inch of rain times the collection factor (which measures your collection efficiency; 80% or 0.8, would be a good efficiency)
- Times the average rainfall
- Divide this figure by 365 to get your daily collection figure.
For more information, see the U.S. Environmental Protection Agency's WaterSense at Work BMP, the Air Force Water Conservation Guidebook, and the General Services Administration Water Management Guide.